Steel laminations have been used for decades in low frequency magnetic components, i.e. components which are subject to magnetic flux. The design of stacked magnetic components must take into account that the magnetic flux is confined in planes parallel to the sheet surfaces. Additionally, there are difficulties with miniaturisation and waste materials with steel laminations which can be important for some type of electric motors.
Since the beginning of the century, iron powder has been used for the production of magnetic components. Powder metallurgy offers the possibility of controlling the spatial distribution of the magnetic flux and allows practically full utilisation of materials even for the manufacture of complicated shapes. Recent advances in powder metallurgy offer new opportunities in the design of electromagnetic components. Several authors have shown the advantages of using iron/resin composites especially for applications in the medium and high frequency ranges.
The required mechanical and magnetic properties of a material depend of the application. For low frequency applications (50-60 Hz typically), the magnetic induction and the permeability must be as high as possible and the core losses must be minimised. Permeability is strongly influenced by the effective length of distributed air-gaps in iron/resin composites. Previous works have shown that permeability decreases dramatically when the resin content increases in iron/resin composites. The resin content should therefore be minimised to keep the permeability high in these materials.
When a magnetic material is exposed to an alternating magnetic field, it dissipates energy. The power dissipated under an alternating field is defined as core loss. The core losses are mainly composed of hysteresis and eddy current losses. Hysteresis losses are due to the energy dissipated by the domain wall movement. The hysteresis losses are proportional to the frequency and are mainly influenced by the chemical composition and the structure of the material. Some currents (i.e., eddy currents) are induced when a magnetic material is exposed to an alternating magnetic field. These currents lead to an energy loss through Joule (resistance) heating. Eddy current losses are expected to vary with the square of the frequency, the square of the powder diameter and inversely with the resistivity. Thus the relative importance of the eddy current losses depends on the material and increases as the frequency increases. At 60 Hz, the losses are mainly constituted of hysteresis losses in iron/resin composites. In contrast, in the case of sintered iron components, eddy currents are the most important part of the losses at the same frequency.
Uncoated iron powders are currently used to make sintered parts for DC magnetic applications. Sintered parts have low resistivity and are generally not used in AC applications. For applications in alternating magnetic field, a minimal threshold resistivity is required. Unsintered iron powder (green compact) has good magnetic properties, acceptable resistivity and can be used for low frequency magnetic applications. However, green compacts have low mechanical properties which will keep these materials off many applications. In AC applications, coated powders or powder mixes containing insulating resins are generally used. The dielectric is used to insulate and bind the magnetic particles together. Ward et al., U.S. Pat. No. 5,211,896 presented a review of the techniques to electrically insulate particles with coatings. A wide range of organic and inorganic insulating binders has already been used. Double coatings have also been used (Roseby, U.S. Pat. No. 1,789,477; Katz, U.S. Pat. No. 2,783,208; Rutz, U.S. Pat. No. 5,063,011; Soileau, U.S. Pat. No. 4,601,765) for applications in alternating magnetic fields.
T. Werber (Joining of Metallic Grains by Thermal Oxidation, J. Novotny and W. Weppner eds., Non-stoichiometric Compounds Surfaces, Grain Boundaries and Structural Defects, 547-556, 1989) studied the mechanism of formation of intergranular oxide joints in uncompacted iron powder in air at 400-800.degree. C. Oxide layers of identical chemical compositions (e.g. Fe.sub.2 O.sub.3) exist on both sides of the contact interface. The external (last formed) lattice planes are only partly ordered and the contact on the interface between growing oxide surfaces is very close. This generates an ordering process at the interface and the formation of a grain boundary between crystallites consisting of the same oxide. Therefore, the interparticle joint is a cohesive bond created by grain boundary formation. The oxide located between the remainders of both initial metal particles can be treated as a monolithic layer common to both particles.
Y. P. Orekhov et al. (Some Properties of a Soft Magnetic Material from Oxidised Iron Powder, Soviet Powder Metallurgy and Metal Ceramics, 15, No. 9 (September 1976), 706-710), studied the effect of the sintering temperature and atmosphere on the magnetic and electric properties of soft magnetic materials fabricated from oxidised iron powder. They showed that the electrical resistivity decreases as the sintering temperature varies from 350.degree. C. to 750.degree. C. (sintering time 0.5 h, steam atmosphere). They associate the variation of the electrical resistivity to changes in the composition and properties of the oxide. Using x-ray diffraction, the authors identified the oxide present at the surface of the powder as magnetite. As the sintering temperature increases, the diffusion of iron and oxygen ions increases in the oxide. The diffusion of oxygen ions in magnetite is slow compared to the diffusion of iron and the concentration of the Fe.sup.2+ rises as the temperature increases. The presence of Fe.sup.2+ facilitates the ionic exchange in the oxide layer between the oxide layer between the iron powder and then reduces the electrical resistivity of the material. These explanations do not take into account possible growing oxide thickness during the sintering.
The oxide naturally present at the surface of the iron powder insulates the particles and the oxide formed during the thermal oxidation treatment binds the particles together. If the sintering temperature is low, the sintering time is short and the atmosphere is appropriate, the insulative oxide layer between iron particles will be continuous and effective to provide parts with improved electrical resistivity compared to sintered iron, and improved mechanical strength compared to untreated iron powder compacts. This is important in soft magnetic material fabricated by P/M (powder metallurgy), intended for AC applications.
It is well known that dielectromagnetics containing iron and resins (thermosets or thermoplastics) such as those described above have very low eddy current losses and perform well in alternating magnetic fields. However, at low frequencies, e.g., 60 Hz, the amount of insulating material must be kept as low as possible in order to improve the ease of magnetisation. It is also known, as it is the case for most of the metal powders, that iron powder is naturally covered by a thin oxide layer which increases significantly the resistivity of bulk iron. For instance, the resistivity of green iron compacts is usually more than two orders of magnitude higher than for cast iron. However, it has not been expected that iron powder compacts can be used at low frequencies without any insulating material. It has been believed that eddy current losses are too high and that the mechanical strength is too low.
It is an object of the invention to develop a method of treatment of iron compacts to obtain the necessary magnetic and mechanical properties of the compacts for magnetic applications at low frequencies.
It is another object of the invention to provide a method to produce powder parts, or elements, with properties offering a compromise between the properties of sintered parts (high permeability) and dielectromagnetics composed of iron powder and resin (low eddy current losses).
It is also an object of the invention to provide powder compacts having the above-described advantageous properties.
Where iron powder is referred to, it is understood that another suitable ferrous alloy may be substituted for pure iron. It is also recognized that entirely pure, non-oxidized iron is practically non-feasible in an industrial environment.